Several traditional methods exist to provide overcurrent protection for distribution equipment. Some of these methods include a distribution fuse cutout with an expulsion fuse link, a distribution cutout in combination with a back-up current limiting fuse (CLF), internal expulsion and current limiting fuses such as completely self protected transformers, and single general-purpose CLF's in a cutout. Each of these methods have their inherent advantages and disadvantages.
The expulsion fuse in a fuse cutout is inexpensive, but provides no energy limiting ability. There has been concern at many utilities about the hot particles and gases that are ejected when expulsion fuses operate. This is particularly dangerous when a lineman is on the pole and closes a cutout into a fault.
The use of two fuses in series allows the replacement of only the unit that has interrupted the overcurrent, thus saving the cost of replacing the intact fuse. The expulsion fuse in this combination is sized to blow on low currents. Only when the available fault current is high, does the more expensive back-up current-limiting fuse operate. The distribution cutout also provides a convenient means of disconnection for the transformer. The disadvantage of this two-fuse method is in the installation space required, and the necessity to stock and carry both of these types of fuses. Also, the venting problem is not entirely eliminated and there is no indication of the operation of the back-up CLF. Also, back-up CLF's are prone to eventful failure if they become damaged and operated on a current below their minimum interrupting rating.
Completely self-protected (CSP) transformers offer a version of the two-fuse method, with the CLF located inside the transformer tank. The internal CLF provides a more compact installation, but the CLF cannot be easily accessed. Some utilities using CSP overhead transformers, however, are unhappy with some aspects of these units. In particular, they would like to have the internal fuses, which are currently mounted inside the transformer tank, accessible for replacement. Also, some utilities find the internal molded case breakers are prone to nuisance blowing and do not allow the utility to emergency load their transformers. At the same time these utilities appreciate the compactness and ease of installation of the CSP units. CSP transformers are particularly desirable in voltage conversion applications when use of the original poles does not allow space for a cutout and back-up CLF to be mounted.
Where space is limited and the presence of an expulsion fuse is undesirable, a single cutout-mounted general-purpose fuse is sometimes preferred. The general-purpose CLF is more compact, but the entire expensive unit must be replaced whenever the fuse operates, even though it may have only been required to interrupt a low current that could have been interrupted by an inexpensive fuse link.
To date there has not been a single solution which address the needs for a compact, inexpensive cutout-mounted fuse which is non-venting and replaceable.